1. Field of the Invention
The present invention relates to a Fresnel lens sheet for use in a rear projection screen.
2. Description of Related Art
A rear projection screen is used for a rear projection type display or the like, and comprises: a Fresnel lens sheet that will be positioned on the light source side; and a lenticular lens sheet that will be positioned on the viewer's side. Such a rear projection screen is mounted in the frame of a rear projection type display or the like, with the Fresnel lens sheet and the lenticular lens sheet being in contact with each other.
FIG. 9 is a view showing a conventional Fresnel lens sheet. As shown in FIG. 9, a Fresnel lens sheet 10′ has a Fresnel lens part 13 that contains a plurality of lenses 11 formed on one plane. The shape of each lens 11 contained in the Fresnel lens part 13 is determined mainly by the lens angle θ and the non-lens angle α; of these angles, the lens angle θ is determined by the optical properties required for the Fresnel lens part 13. Namely, the lenses 11 continuously increase in lens angle θ as the lens position gets away from the center toward the outer edge of the Fresnel lens part 13, and owing to this increase in lens angle θ, the apexes 11a of the lenses 11 become sharper as the lens position gets away from the center toward the outer edge of the Fresnel lens part 13.
As shown in FIG. 10, such a Fresnel lens sheet 10′ constitutes, together with a lenticular lens sheet 20 (and, if necessary, a front panel (not shown in the figure)), a rear projection screen 30, which is mounted in the frame (not shown in the figure) of a rear projection type display or the like. In order to prevent production of fuzzy images that is caused by separation between the Fresnel lens sheet 10′ and the lenticular lens sheet 20, the lenticular lens sheet 20 is curved along the Fresnel lens sheet 10′. It is therefore possible to keep the Fresnel lens sheet 10′ and the lenticular lens sheet 20 being in close contact with each other.
As mentioned above, in the Fresnel lens sheet 10′, the apexes 11a of the lenses 11 in the Fresnel lens part 13 become sharper as the lens position gets away from the center toward the outer edge of the Fresnel lens part 13. Therefore, at points B at which the Fresnel lens sheet 10′ and the lenticular lens sheet 20 come in contact with each other, the apexes 11a of the lenses 11 in the Fresnel lens part 13 are pressed by the lenticular lens part 21, as shown in FIG. 10. Consequently, the apexes 11a of the lenses 11 are collapsed and deformed, which tends to lead to the distortion of imaging light. This “collapse” problem often occurs in area C (the hatched area in FIG. 11) which is the marginal part of the rear projection screen 30 along its four sides, the width of this part being not more than 200 mm, and in which the apexes 11a of the lenses 11 in the Fresnel lens part 13 are sharper.
Further, in the Fresnel lens sheet 10″, the apexes 11a of the lenses 11 in the Fresnel lens part 13 become sharper as the lens position gets away from the center toward the outer edge of the Fresnel lens part 13, as mentioned above, so that at points D at which the Fresnel lens sheet 10′ and the lenticular lens sheet 20 come in contact with each other, the apexes 11a of the lenses 11 in the Fresnel lens part 13 and the lenticular lens part 21 are, as shown in FIG. 12, rubbed with each other because of vibration that is caused during transportation, for example. As a result, the lenticular lens part 21 is partly abraded by the apexes 11a of the lenses 11 in the Fresnel lens part 13 and is thus deformed, which tends to lead to the distortion of imaging light. Although this “abrasion” problem occurs on the entire surface of the rear projection screen 30, it occurs more often in the area C (the hatched area in FIG. 11) which is the marginal part of the rear projection screen 30 along its four sides, the width of this part being not more than 200 mm, and in which the apexes 11a of the lenses 11 in the Fresnel lens part 13 are sharper.
In order to solve the above-described “collapse” and “abrasion” problems, attempts have been made to properly control and design the physical properties of molding resins for the Fresnel lens sheet 10′ and for the lenticular lens sheet 20. It is, however, difficult to fully solve the “collapse” and “abrasion” problems by such measures.
On the other hand, such a rear projection screen 30 as is shown in FIGS. 10 to 12 (a rear projection screen 30 comprising a Fresnel lens sheet 10′ and a lenticular lens sheet 20) has the shortcoming that bright-and-dark fringes that appear separately on the Fresnel lens sheet 10′ and on the lenticular lens sheet 20 produce a moiré pattern. Such a moiré pattern tends to appear in the marginal part of the rear projection screen 30 (see symbol M in FIG. 11).
Namely, black stripes (see reference numeral 22 in FIG. 1A) are usually provided periodically on the lenticular lens sheet 20, and owing to these black stripes 22, vertical bright-and-dark fringes appear periodically on the lenticular lens sheet 20 in the horizontal direction. On the other hand, in the Fresnel lens sheet 10′, portion A through which light from a source inherently does not pass exists in each lens 11 in the Fresnel lens part 13, as shown in FIG. 9. Therefore, bright-and-dark fringes appear along the lenses 11 that extend concentrically, for example. Since the radii of the concentric lenses 11 in the Fresnel lens part 13 are greater in the outer edge part of the Fresnel lens sheet 10′, the bright-and-dark fringes appear almost vertically in this part of the Fresnel lens sheet 10′. The vertical bright-and-dark fringes that appear periodically on the lenticular lens sheet 20 and the almost vertical bright-and-dark fringes that appear on the Fresnel lens sheet 10′ produce moiré fringes that are periodically repeating in the horizontal direction.
To solve the above-described “moiré” problem, there has been proposed a method of reducing the appearance of a moiré pattern, in which the ratio of the lens pitch on a Fresnel lens sheet 10′ to that on a lenticular lens sheet 20 is made either “N+0.35 to 0.43” or “1/(N+0.35 to 0.43)” (where N is a natural number of 2 to 12) (see Japanese Laid-Open Patent Publication No. 95525/1984, for example).
Further, in order to control the vertical diffusion of light, a diffusing agent is often incorporated in the substrate of a Fresnel lens sheet 10′, or a lens part called V-type lenticular lenses (lenticular lenses for vertically diffusing light) is often provided on the plane of incidence of a Fresnel lens sheet 10′. In either case, incident light F, light from a source, diffuses in the substrate of the Fresnel lens sheet 10′ to become diffused light G, as shown in FIG. 9, and diffused light H is to strike even the portion A through which the light from a source inherently does not pass. This portion A is, therefore, prevented from getting dark, and, as a result, the appearance of a moiré pattern is reduced.
It is, however, difficult to fully solve the “moiré” problem by any of the above-described conventional means. In particular, in the case where a diffusing agent is incorporated in a Fresnel lens sheet 10′, or where a V-type lenticular lens part is provided on a Fresnel lens sheet 10′, imaging light that has passed through a lenticular lens sheet 20 and that will finally produce an image diffuses while passing through the Fresnel lens sheet 10′, so that the image produced appears fuzzy.
Those Fresnel lens sheets that are described in Japanese Laid-Open Patent Publication No. 249602/1991 are related to the present invention. The apexes of lenses on these Fresnel lens sheets are in the shapes of circular arcs with curvature radii between 1 μm and 10 μm, in the shapes of polygons with curvature radii between 1 μm and 10 μm, or in the shapes of planes with lengths between 1 μm and 10 μm.
However, the purposes of the technique described in the above Japanese Laid-Open Patent Publication No. 249602/1991 are to eliminate the difficulty in releasing a Fresnel lens sheet from a mold and to prevent the apexes of prism lenses from being damaged, and the aforementioned “collapse”, “abrasion” and “moiré” problems cannot fully be solved by this technique. In particular, in the technique described in this Patent Publication, the shape of the apexes of the prism lenses is determined, over the entire Fresnel lens sheet surface, solely by the shape of a tool tip. Therefore, of the light from a source, incident on the Fresnel lens sheet, the light incident on a part of the Fresnel lens sheet (especially on the center portion) tends to become stray light, and this stray light adversely affects the rear projection screen.